Earth's Inner Core Is Solid — Not Liquid — Even Though It's Blistering Hot

• Earth's inner core is solid and blistering hot.

• For decades, scientists have known the inner core is solid thanks to the pioneering work of Danish seismologist Inge Lehmann, who first proposed its existence in 1936 after analyzing how seismic waves behaved following a large earthquake near New Zealand.

• Earth’s inner core is a roughly 1,500-mile-wide sphere made mostly of iron and nickel. It sits inside the planet’s liquid outer core, which itself is nestled beneath the mantle and crust. The inner core formed roughly a billion years ago as Earth’s interior finally began to cool and solidify following its formation.

Deep beneath our feet, farther than any drill could hope to reach, lies a realm as extreme as it is mysterious: Earth’s inner core. To many, this superheated metal sphere at the planet’s heart is one of geology’s greatest mysteries. Is it solid or liquid? Static or dynamic? And just how hot is it down there?

The short answer: Yes, Earth’s inner core is solid, and it’s blistering hot. But it’s also surprisingly complex, and it still harbors many secrets.

Fortunately, recent seismic breakthroughs, including the detection and analysis of elusive shear waves, are reshaping our understanding of this region. The findings suggest that Earth’s inner core is not just solid, but also layered, slowly moving, and perhaps even convecting, like a lava lamp, despite being composed of solid metal.

Earth’s inner core is a roughly 1,500-mile-wide sphere made mostly of iron and nickel. It sits inside the planet’s liquid outer core, which itself is nestled beneath the mantle and crust. The inner core formed roughly a billion years ago as Earth’s interior finally began to cool and solidify following its formation. And today, the inner core still plays a vital role in generating Earth’s global magnetic field, which shields life from solar and cosmic radiation.

For decades, scientists have known the inner core is solid thanks to the pioneering work of Danish seismologist Inge Lehmann, who first proposed its existence in 1936 after analyzing how seismic waves behaved following a large earthquake near New Zealand. She noticed subtle wave reflections that suggested Earth has a solid sphere nested inside a liquid outer core. But it wasn’t until recently that researchers captured direct evidence of shear waves moving through that solid inner core, allowing them to better understand its true structure.

“The detection of shear waves (J-waves) traveling through the inner core is a breakthrough because it reveals specific mechanical properties of this solid region, roughly the size of Pluto,” says Hrvoje Tkalčić, Head of Geophysics at the Australian National University. “Shear waves only travel through solids, and their speed and behavior depend on the material’s rigidity and density.”

So by measuring these waves, researchers can better estimate the stiffness and composition of the inner core. And the results not only confirm the inner core’s solid nature, but also help scientists better understand its formation and evolution over time.

Read More: Earth’s Inner Core Is Slowing Down — and May Not Be Entirely Solid

The inner core is the hottest layer of Earth. 

“If we could peel away the mantle and outer core, we’d behold the inner core: a radiant, solid sphere of iron and nickel, glowing with temperatures near 6,000 degrees Celsius (10,800 degrees Fahrenheit), akin to the Sun’s surface,” says Tkalčić.

That heat is left over from the planet’s formation and is sustained by radioactive decay and pressure from the overlying layers. But despite these extreme temperatures, the intense pressure at those depths prevents the inner core from melting, keeping it solid.

Meanwhile, the surrounding liquid outer core, which is slightly cooler, churns with convective motion thanks to the extreme heat transfer at its boundary with the inner core. This convective motion in the liquid outer core generates Earth’s geodynamo, the mechanism that sustains our planet’s protective magnetic field.

Even though the inner core is solid, it’s not permanently fixed in place. It may rotate at a slightly different rate than the rest of the Earth, and over geologic time, it can distort under pressure.

“On a human lifetime scale, the mantle and inner core appear solid,” Tkalčić says. “But over millions of years, they can flow and deform under immense heat and pressure, much like silly putty, which is firm when squeezed quickly but stretches slowly under gentle force.”

This plasticity opens the door to processes like slow rotation relative to the mantle and even thermal convection, where heat currents can cause the solid inner core to very slowly churn. That subtle motion, as well as structural complexities, could influence how the magnetic field changes over time and how Earth’s long-term tectonic processes are powered.

The precise information unlocked by measuring shear waves also reveals something else unexpected: The inner core may have multiple layers, including what some researchers call the innermost inner core.

“Seismic data suggest a distinct central region, roughly 300 km to 600 km in radius, with different anisotropy compared to the outer inner core,” Tkalčić says. “Hinting at an innermost inner core.” This subtle but detectable layering could be due to differences in crystal alignment, composition, or thermal changes over time, which Tkalčić says could be the result of the core solidifying in distinct phases.

This observed complexity not only challenges the idea of a uniformly solid inner core, Tkalčić says, “[It] refines our understanding of the inner core’s role in Earth’s magnetic field generation and its evolution, indicating it’s not just a static solid but a region with intricate, possibly mobile internal dynamics.”

Despite these advances, many questions about the inner core still remain. For instance, seismic observations, laboratory experiments, and mineral physics estimates still don’t completely agree on the core’s composition or rigidity.

To attempt to solve the puzzle, researchers are embracing a mix of new technologies. High-resolution seismic X-ray imaging, machine learning, and even next-generation neutrino detectors may help them map the inner core in greater detail. And at the same time, powerful computer simulations, known as ab initio models, are helping predict how iron and nickel behave under conditions no lab can replicate.

“Innovative methods such as the correlation wavefield that we employed to detect J-waves in the inner core hold the keys to success and the way forward,” Tkalčić says. “Coupling these with machine learning to analyze vast seismic datasets can enhance detection of faint signals, revealing dynamic processes such as convection, dynamic topography, or differential rotation.”

Though we’ll never visit it firsthand, Earth’s inner core remains one of the most teasing frontiers of science — a completely hidden sphere, the temperature of the Sun that shapes the world above, not below.

“Exploring Earth’s inner core is as captivating as gazing at distant planets and stars, both drawing us to ponder our planet’s evolution, the Universe’s history, and our place within it,” Tkalčić says. “Like a star, it shapes its surroundings, driving the geodynamo that generates Earth’s magnetic field, shielding life from cosmic threats. Studying this inner ‘star’ not only reveals Earth’s ancient past, but also mirrors humans’ quest to understand the cosmos.”

Read More: Earth’s Inner Core Is Growing Lopsided

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Australian National University. The Earth's Inner Core

• Science Daily. Earth's inner core was formed 1-1.5 billion years ago

• NASA. Earth’s Magnetosphere: Protecting Our Planet from Harmful Space Energy

• Harvard University. Dynamo theory

• Scientific Reports. Imaging the top of the Earth’s inner core: a present-day flow model

• Brown University. Traditional vs ab initio models

Jake Parks is a freelance writer and editor who specializes in covering science news. He has previously written for Astronomy magazine, Discover Magazine, The Ohio State University, the University of Wisconsin-Madison, and more.